1. Field of the Invention:
The present invention relates to probes for sampling fluids located in a wall confined region, such as a pipe. More particularly, it relates to remote sampling probes for sampling stack emission gases.
2. Discussion of the Prior Art:
Sampling probes generally comprise a sample tube extending into a wall confined region where the fluid of interest (hereinafter referred to as the primary fluid) is located, usually as a flowing stream of gas. If the pressure in the confined region is high enough, the primary fluid will flow through the sample tube to a detecting station. If the pressure in the confined region is insufficient, a vacuum is applied to the sample tube to withdraw a portion of the primary fluid from the confined region and to conduct it to a detecting station containing a measuring device, such as a process stream photometer, designed to measure the concentration of a constituent or constituents of the primary fluid.
Often the point at which the sampling takes place is remote from the measuring device. Such is the case when measuring stack emission gases using ground based equipment. Since the measured concentration of the various constituents of the primary fluid is dependent on the temperature of the fluid, the temperature of the fluid reaching the measuring device should be equal, or at least proportional, to that of the fluid in the confined region. Hence, when the measuring device is remotely located from the sample point, the conduit connecting the sample probe to the measuring device is maintained at a controlled temperature, commonly by an electrically heated conduit. However, it is difficult to electrically control the temperature of the entire sample probe, particularly when that probe contains a calibration fluid inlet.
Furthermore, since emisson gases and other primary fluids often contain particulate material which would either clog the sample conduit or interfere with the measurement, the end of the sample probe which extends into the confined region is usually capped with a filter. These filters have a tendency to become clogged with the particulate matter in the primary fluid, and this effects the precision of the measurement.